Brackets designed to retain arch wires will be used as examples to describe the material composition and characteristics of prior art orthodontic appliances. Brackets are ordinarily formed of stainless steel, titanium, ceramic or plastic materials. Brackets made of stainless steel generally have superior strength characteristics and can be more easily manufactured than other brackets. However, the presence of chromium and nickel in the stainless steel has been known to cause allergic reactions in some patients.
Alloyed titanium brackets have superior corrosion resistance properties. Pure titanium brackets generally lack sufficient strength for orthodontic use. The Ti--6Al--4V titanium alloy has sufficient strength, but includes a toxic vanadium element. Moreover, the presence of this alloy inside a patient's mouth with other metals may cause a galvanic cell effect with the risk of metal elution. In addition, the alloy is expensive and not well suited to manufacturing processes.
Ceramic brackets have an aesthetically pleasing appearance, but because they are harder than tooth enamel, they may cause tooth surfaces to become scratched during chewing. In addition, tooth enamel may be damaged when ceramic brackets are removed from tooth surfaces during or after completion of orthodontic treatment due to the stiffness of the material. Also, a practitioner may be forced to grind tooth surfaces with a diamond tool to remove any portions of ceramic brackets remaining on the tooth surfaces.
Plastic brackets also have an aesthetically pleasing appearance. Plastic brackets however do not have substantial mechanical strength and may break or deform when external forces are applied to the bracket by arch wires during treatment. Also, plastic brackets have relatively high rates of water absorption and tend to discolor easily from food colorants. Moreover, there is substantial frictional resistance to smooth movement of arch wires relative to plastic brackets, which may lengthen the period of orthodontic treatment.
With these prior brackets, when changes and adjustments are required during orthodontic treatment, ordinarily either the arch wires are removed from the brackets, bent, and reinstalled to continue orthodontic treatment, or the brackets are removed from the tooth surfaces and reattached in desired positions. Thus, a substantial amount of work is required by a practitioner during orthodontic treatment.
The drawbacks of prior art brackets are also applicable at least to some degree to other orthodontic appliances.